In many industries, like the petrochemical and chemical industries, contact of reaction fluids with a catalyst in a reactor under suitable temperature and pressure conditions effects a reaction between the components of one or more reactants in the fluids. Most of these reactions generate or absorb heat to various extents and are, therefore, exothermic or endothermic. The heating or chilling effects associated with exothermic or endothermic reactions can positively or negatively affect the operation of the reaction zone. The negative effects can include among other things: poor product production, deactivation of the catalyst, production of unwanted by-products and, in extreme cases, damage to the reaction vessel and associated piping. More typically, the undesired effects associated with temperature changes will reduce the selectivity or yield of products from the reaction zone.
Many arrangements seek to overcome the negative effects of heating or chilling by supplying heat to the reaction. More traditional methods employ multiple stages of heating or cooling between adiabiatic reaction stages. Other methods use in situ heating via simultaneous reactions or indirect heat exchange to maintain an isothermal or other temperature with the reaction zone. U.S. Pat. No. 5,525,311 provides an example of indirect heat exchange with a heat exchange fluid to control the temperature profile within a reaction zone.
A variety of processes can employ indirect heat exchange with a reaction zone to control temperature profiles within the reaction zone. Common examples are of hydrocarbon conversion reactions include: the aromatization of hydrocarbons, the reforming of hydrocarbons, the dehydrogenation of hydrocarbons, and the alkylation of hydrocarbons.
Briefly, in catalytic reforming, a feedstock is admixed with a recycle stream comprising hydrogen and contacted with catalyst in a reaction zone. The usual feedstock for catalytic reforming is a petroleum fraction known as naphtha. Reforming may be defined as the total effect produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics, dehydrogenation of paraffins to yield olefins, dehydrocyclization of paraffins and olefins to yield aromatics, isomerization of n-paraffins, isomerization of alkylcycloparaffins to yield cyclohexanes, isomerization of substituted aromatics, and hydrocracking of paraffins. A catalytic reforming reaction is normally effected in the presence of catalyst particles comprised of one or more Group VIII noble metals (e.g., platinum, iridium, rhodium, palladium) and a halogen combined with a porous carrier, such as a refractory inorganic oxide. The halogen is normally chlorine. Alumina is a commonly used carrier. Further information on reforming processes may be found in, for example, U.S. Pat. No. 4,119,526 (Peters et al.); U.S. Pat. No. 4,409,095 (Peters); and U.S. Pat. No. 4,440,626 (Winter et al), the contents of which are herein incorporated by reference.
Catalytic dehydrogenation is another example of an endothermic process. In catalytic dehydrogenation, a feedstock is admixed with a recycle stream comprising hydrogen and contacted with catalyst in a reaction zone. Feedstocks for catalytic dehydrogenation are typically petroleum fractions comprising aromatic of paraffinic hydrocarbons. The dehydrogenation of ethyl benzene to produce styrene is well known. Paraffinic feedstock ordinarily have from about 3 to about 18 carbon atoms. Particular feedstocks will usually contain light or heavy paraffins. A catalytic dehydrogenation reaction is normally effected in the presence of catalyst particles comprised of one or more Group VIII noble metals (e.g., platinum, iridium, rhodium, palladium) combined with a porous carrier, such as a refractory inorganic oxide. Alumina is a commonly used carrier. Dehydrogenation conditions include a temperature of from about 400.degree. to about 900.degree. C., a pressure of from about 0.01 to 10 atmospheres and a liquid hourly space velocity (LHSV) of from about 0.1 to 100 hr.sup.-1. Generally, for normal paraffins, the lower the molecular weight the higher the temperature required for comparable conversions. The pressure in the dehydrogenation zone is maintained as low as practicable, consistent with equipment limitations, to maximize the chemical equilibrium advantages. The preferred dehydrogenation conditions of the process of this invention include a temperature of from about 400-700.degree. C., a pressure from about 0.1 to 5 atmospheres, and a liquid hourly space velocity of from about 0.1 to 100 hr.sup.-1.
The effluent stream from the dehydrogenation zone generally will contain unconverted dehydrogenatable hydrocarbons, hydrogen and the products of dehydrogenation reactions. This effluent stream is typically cooled and passed to a hydrogen separation zone to separate a hydrogen-rich vapor phase from a hydrocarbon-rich liquid phase. Generally, the hydrocarbonrich liquid phase is further separated by means of either a suitable selective adsorbent, a selective solvent, a selective reaction or reactions or by means of a suitable fractionation scheme. Unconverted dehydrogenatable hydrocarbons are recovered and may be recycled to the dehydrogenation zone. Products of the dehydrogenation reactions are recovered as final products or as intermediate products in the preparation of other compounds. Additional information related to the operation of dehydrogenation catalysts, operating conditions, and process arrangements can be found in U.S. Pat. Nos. 4,677,237; 4,880,764 and 5,087,792, the contents of which are hereby incorporated by reference.
Another process example are processes for the production of hydrogen and carbon oxides by reforming methane in the presence of steam or carbon oxides have been practiced for many years. The steam reforming process is particularly well known and involves passage of a mixture of feedstock and steam over a steam reforming catalyst. Typical steam reforming catalyst comprises nickel and may include cobalt on refractory supports such as alpha alumina or calcium aluminate. The strong endothermic nature of the primary steam reforming reaction requires a supply of heat to maintain the reaction. Those skilled in the art routinely balance the endothermic heat requirements of the primary reforming with a partial oxidation of hydrocarbons to provide a secondary reforming reaction that supplies heat for the primary reforming stage and generates additional synthesis gas. Extensive and highly developed teachings detail methods of indirectly exchanging heat between primary and secondary reforming. The operation of an adiabatic reformer for synthesis gas production is shown in U.S. Pat. No. 4,985,231. U.S. Pat. No. 5,300,275 sets forth another basic arrangement that uses a secondary reforming reaction to supply hot gas for heating the primary reforming reaction. Patents U.S. Pat. Nos. 4,810,472; 4,750,986; and 4,822,521 disclose particular arrangements of heat exchange reactors that indirectly exchange heat between hot gases from the secondary reforming stage and the primary reforming stage. U.S. Pat. No. 4,127,389 shows a variety of tube chamber designs for supplying heat to a primary reforming reaction from a secondary reforming reaction zone. As established by the above referenced patents, the art currently relies exclusively on tube arrangements, and most commonly relies on double walled tubes referred to as "bayonet tubes, for exchanging heat between the primary and secondary reforming zones. The geometry of tubular reactors poses layout constraints that require large reactors and vast tube surface to achieve high heat transfer efficiencies.
Other process applications accomplish indirect heat exchange with thin plates that define channels. The channels alternately retain catalyst and reactants in one set of channels and a heat transfer fluid in adjacent channels for indirectly heating or cooling the reactants and catalysts. Heat exchange plates in these indirect heat exchange reactors can be flat or curved and may have surface variations such as corrugations to increase heat transfer between the heat transfer fluids and the reactants and catalysts. Many hydrocarbon conversion processes will operate more advantageously by maintaining a temperature profile that differs from that created by the heat of reaction. In many reactions, the most beneficial temperature profile will be obtained by maintaining substantially isothermal conditions. In some cases, a temperature profile directionally opposite to the temperature changes associated with the heat of reaction will provide the most beneficial conditions. For such reasons it is generally known to contact reactants with a heat exchange medium in cross flow, cocurrent flow, or countercurrent flow arrangements. A specific arrangement for heat transfer and reactant channels that offers more complete temperature control can again be found in U.S. Pat. No. 5,525,311; the contents of which are hereby incorporated by reference. Other useful plate arrangements for indirect heat transfer are disclosed in U.S. Pat. Nos. 5,130,106 and 5,405,586.
Supplying the heat for indirect heat exchange can be done directly by the combustion of a fuel. The use of catalysts to control the direct combustion of a fuel for heating purposes is well known. U.S. Pat. No. 5,328,359 shows an arrangement of coated plates for the combustion of fuel in a multistage combustor. The '359 patent deals with the problem of overcoming localized hot spots on the plates that can deactivate the catalyst.
It is an object of this invention to improve the efficiency of heating reactants in a process that uses direct fuel combustion.
It is a further object of this invention to more closely match the heating requirements of the reaction with the combustion of the fuel.
Another object of this invention is to control the introduction of combustion reactants into a system for in-situ heating of reactants in a reaction zone by combustion of fuel in a heating zone that communicates thermally with the reaction zone.